1. Field
The following description relates to a passive optical network (PON), and more particularly, to optical amplification technologies in a PON.
2. Description of the Related Art
A passive optical network (PON) is a subscriber network that connects a central office and a subscriber using a point-to-multipoint topology and is cost effective compared to a structure having a point-to-point topology since less central office systems and optical cables are required. A time division multiplexing-passive optical network (TDM-PON), for example, Ethernet PON (EPON) and Gigabit-Capable PON (GPON), uses one wavelength for upstream traffic and another wavelength for downstream traffic to connect a central office to subscribers. In addition, the TDM-PON uses one wavelength for upstream traffic and another wavelength for downstream traffic to connect a number of subscriber devices and a central office system. Thus, when multiple subscriber devices simultaneously transmit an upstream signal of the same wavelength to the central office system, the central office system cannot handle the situation. To address this problem, the TWDM-PON is used in order to transmit an upstream signal in burst mode. That is, each subscriber device transmits an upstream optical signal only at the transmission time that has been allocated to it by the central office system. If ‘N’ number of subscriber devices are connected to one central office system, subscriber device 1 and subscriber device N cannot simultaneously transmit optical signals, but rather, consecutively transmit the optical signals to the central office system. Such a manner of consecutive signal transmission is referred to as “burst transmission mode”, whereby at this time, some of the subscriber devices that are connected to the central office system may not operate for reasons such as power saving. In such cases, certain segments may be void of any optical signals which are lapses in the stream.
Time and wavelength division multiplexing-passive optical network (TWDM-PON) has been developed by advancing an existing TDM-PON. The TWDM-PON is a hybrid passive optical network that combines both TDM and WDM technologies. The TWDM-PON may allow for the expansion of transmission capacity in proportion to the number of optical wavelength channels without modifying the optical distribution network used for an existing TDM network. In the TWDM-PON, upstream optical signals are transmitted and received using TDM technology, and thus they are transmitted in burst mode.
Hence, an optical amplifier in an optical line terminal (OLT) used for implementing burst mode in the TWDM-PON by amplifying an upstream signal may need to minimize signal distortion during the amplification of optical signals, and especially, needs to have a sufficient level of optical power tolerance for receiving burst-mode optical signals of different sizes. When a burst-mode signal is fed into a general optical amplifier, initially there is a sufficient increase of gain; but then the gain gradually decreases as population inversion of the optical amplifier decreases over time, which is an inherent physical characteristic of an optical amplifier. The gain decreasing time of an optical amplifier depends on the energy conversion capacities of the materials that constitute a gain medium.
Korean Published Patent Application No. 10-2009-0004913 relates to a bi-directional optical amplifier to amplify continuous downstream signals and burst-mode upstream signals as optimally as possible. However, said Korean patent application only discloses the amplification of downstream and upstream signals using a transponder and thus cannot provide any solution to effectively reduce distortion of optical signal.